The present invention relates to an assembly of a fan and a fan drive, and more particularly, to an improved structure and method relating to the assembling of the fan and fan drive.
It will become apparent to those skilled in the art from the subsequent description that the structure and assembly method of the present invention may be utilized advantageously with various types of fans, as well as with various types of fan drives. However, the invention is especially useful for assembling a radiator cooling fan, for cooling a vehicle engine, to a viscous fan drive, and the invention will be described in connection therewith. The invention is also especially useful when the fan is of the type having a hub portion and fan blades molded integrally therewith from a plastic material, and an annular metal spider having its outer periphery attached to the hub portion and its inner periphery attached to the housing of the viscous fan drive. Such a fan and fan drive assembly is illustrated and described in U.S. Pat. No. 4,169,693, assigned to the assignee of the present invention and incorporated herein by reference.
Typically, the viscous fan drive and the radiator cooling fan have been manufactured and shipped separately to the vehicle OEM assembly plant (i.e., either the engine assembly plant or the final vehicle assembly plant).
It has been conventional practice to provide the housing of the viscous fan drive with a plurality (typically, four) of threaded bores. Then, on the moving assembly line at the vehicle OEM assembly plant, the assembler must position the fan on the fan drive, insert a bolt in each threaded bore, and "torque" or tighten each of the bolts to the appropriate level of tightness. The tightening of the bolts is normally done with a pneumatic (air) gun. It should be noted that the assembler typically has less than about one minute to assemble the fan to the fan drive and then put that assembly on the water pump, either by bolting it to the water pump or threading it onto the water pump shaft.
Although the above-described assembly routine has been generally satisfactory, in terms of the product of the final assembly, the occurrence of bolt strippage problems in the assembly plant has been undesirably high. One reason for the occurrence of bolt strippage is that the assembler, in order to keep pace with the movement of the assembly line, may actually start the bolt into the fan drive housing using the air gun, which may result in cross-threading of the threaded bores, unless the bolt and the bore are in nearly perfect alignment, which is difficult on the moving assembly line. Occasionally, the assembler will be given the wrong bolts (for example, ones which are too short), and the attempt to insert the incorrect bolts (or overtorquing bolts which are too short) results in damage to the threads of the fan drive housing. In either case, the result is that one or more of the threaded bores in the fan drive housing is stripped, and the fan drive either becomes a warranty return, and is shipped back to the viscous fan drive manufacturer, or is merely discarded, representing a loss for the vehicle OEM.
The above-described problems are somewhat common on the typical fan and fan drive assembly requiring four bolts, but there is obviously even greater opportunity for bolt strippage and damage to the fan drive on some of the newer, larger fans and fan drives which require six bolts.